I. Field of the Invention
The present invention relates generally to the fields of biology, chemistry and medicine. More particularly, it concerns derivatives of potassium channel inhibitors, including derivatives of 4-aminopyridine, and methods of making and using thereof, including for the treatment and medical imaging of neurodegenerative conditions.
II. Description of Related Art
With nearly 400,000 people affected in the U.S. and 2.5 million worldwide, Multiple Sclerosis (MS) is the most common neurodegenerative condition in young adults (Calabresi, 2007). The progressive demyelination of neurons in the brain leads to diverse neurological symptoms. Myelin is the multilayered membrane that surrounds most axons of the central and peripheral nervous systems and is essential for the propagation of rapid nerve impulses. In people with MS, the myelin sheath that normal covers the axons is lost and this leads to aberrant leakage of potassium ions from the axon and improper impulse conduction.
One approach to treat MS or to mitigate the symptoms associated with MS is to block potassium channels to reduce the leakage of potassium ions, thus enhancing impulse conduction. In January 2010, the FDA approved 4-aminopyridine (4-AP), as a therapy for MS (Ampyra, Acorda Therapeutics, Inc., 2010). 4-AP is a relatively selective blocker of Kv1 family of K+ channels (Wulff et al., 2009). By blocking K+ channels, impulse conduction along the axon is partially restored and symptoms ameliorate.
To develop new neuroprotective therapies for MS or other neurodegenerative diseases, it is essential to have proper tools to diagnose and assess disease progression.
According to CDC around 1.74 million people sustain a traumatic brain injury in the U.S. each year. Most of these injuries are mild (75%) and certain populations are at a higher risk: men aged 0-4, 15-19 and over 60 as well as military personnel and people engaged in contact sports. Recent studies have shown that even mild TBIs can have serious consequences later in life. TBI has been linked to depression, anxiety, substance abuse and suicide. All these reasons make screening for TBI particularly important.
Currently, the diagnosis of TBI is based on clinical evaluation aided by Computed Tomography (CT) or Magnetic Resonance Diffusion Tensor Imaging (MR-DTI). CT scans are very useful for detecting mass lesions and fractures but do not allow visualization of mild TBIs. More recently, MR-DTI has emerged as a sensitive method to evaluate white matter integrity in TBI but it too can be difficult to interpret.
During TBI, compression or stretching of the brain often causes damage to axons and/or the myelin sheath. Oligodendrocytes, the cells responsible for producing and maintaining myelin, have also been shown to be sensitive to TBI (Flygt et al., 2013; Sharp and Ham, 2011; Morey et al., 2012). Oligodendrocyte injury results in axonal demyelination. In addition to facilitating raping nerve conduaction velocities, myelin provides axonal protection, such that demyelinated axons are prone to degeneration. Therefore, TBI-induced damage to myelin and/or oligodendrocytes likely contributes to the acute and long-term clinical manifestations of TBI.
Axonal proteins are compartmentalized in myelinated axons, with the voltage-gated sodium channels concentrated at the unmyelinated node of Ranvier and the rectifying potassium (K+) channels residing under the myelin sheath (Waxman and Ritchie, 1993). Following demyelination, like that which occurs in multiple sclerosis (MS) and TBI, the K+ channels become exposed and leaky. In 2010, the FDA approved 4-amino-pyridine (4-AP, Ampyra®) as a drug to improve symptoms in people with MS. 4-AP is a K+ channel blocker that binds to the exposed channels on demyelinated axons, which reduces the aberrant efflux of K+ ions and enhances neuronal conduction. Since 4-AP selectively targets K+ channels that have become uncovered as a result of demyelination we propose to test its usefulness as a tracer for demyelinated axons.
Not much is known about the role of axonal K+ channels and the effects of 4-AP in TBI. However, there have been numerous studies looking at the effects of 4-AP after Spinal Cord Injury (SCI; Blight et al., 1989; Blight et al., 1991; Hayes et al., 1993; Fehlings and Nashmi, 1996; Gruner and Yee, 1999). Similarly to TBI, SCl is an injury to the CNS that occurs after a violent impact. Depending on the location and severity of the injury the symptoms can vary from partial loss of movement and sensation (incomplete injury) to complete loss. In cases of incomplete injury, 4-AP has been shown to enhance neuronal conduction through injured areas both in animals and in humans (Blight et al., 1989; Blight et al., 1991; Hayes et al., 1993). In addition, injured spinal cord areas have been shown to have higher pharmacological sensitivity to 4-AP (Fehlings and Nashmi, 1996), which agrees with our hypothesis that K+ channels on demyelinated fibers are more accessible and suggest the potential of using radioactive 4-AP to map injured areas. The similarities between TBI and SCI in etiology and at the histopathological level justify evaluating 4-AP based PET tracers for TBI. In addition, if 4-AP is found to localize to injured areas in TBI it could also be useful for restoring function/ameliorating symptoms in TBI patients. Fluorine-18 is the preferred isotope for PET imaging because its long half-life allows for off-site production and commercialization. In addition, its low positron energy gives higher resolution than for example carbon-11. We have also shown that these fluorinated molecules have very similar properties to 4-AP both in vitro and in vivo indicating that fluorination does not disrupt its properties and therefore these molecules could be used as surrogates of 4-AP.
Thus, there is a pressing need for new, accurate methods to evaluate and diagnose TBI.